Copepods.
Copepods are extremely abundant animals in both freshwater and marine habitats, and the introductory notes on the ecological importance of the smaller crustacaea apply particularly to them.
The free-living copepods fall into three orders:
- Cyclopoidea.
Includes the well known Cyclops, of which there are over a hundred species. Moderately long secondary antennae, and the females carry twin egg-sacs.
- Calanoidea.
Characterized by long secondary antennae, and in the females, a single egg-sac. Includes Diaptomus.
- Harpacticoidea.
Much smaller than Cyclops and Diaptomus, and usually found foraging on submerged plants. Their small antennae do not enable them to swim.
Not illustrated here.
Also not illustrated are the numerous highly adapted copepods which are parasitic upon fish.
Cyclops.
The various species of Cyclops are very numerous in freshwater locations during the summer months, and are usually among the much reduced fauna in these same locations during the winter. The main task facing the microscopist is that of catching a specimen for examination under the microscope, as they dart very quickly away when any attempt is made to catch them. Specimens collected from the open waters of a pond or lake often have more transparent cuticles than those at the water's edge, and provide a better view of internal structures.
Here is a diagram of Cyclops.
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This adult female Cyclops is typical of the specimens found in most freshwater habitats. The twin egg-sacs, when present, are the main distinguishing feature of the Cyclopoidea. The colour of this specimen is also fairly typical, although occasionally they are found coloured in deep reds, blues or purple.
Darkfield, x100. |
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This mature female Cyclops is seen from the side, and one of the eggs of the egg-sac has just hatched into the nauplius larva seen on the right. Immediately after hatching and before their cuticle hardens, the larvae take up water and grow rapidly in size. They undergo a series of moults to arrive at the adult condition with a group of feeding appendages at the head end, five pairs of swimming feet and a body showing a distinct division into cephalothorax and abdomen.
Darkfield, x100. |
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A high-power picture of a Cyclops nauplius larva similar to the newly-hatched nauplius in the picture above, showing the fat globules which are the energy reserves of the larva until it begins to take in its own food.
Darkfield, x400.
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The same nauplius as the picture above, in brightfield illumination. To get some idea of the dramatic differences in the size and structure of the nauplius before and after the first moult, check this diagram taken from an old textbook.
Darkfield, x400.
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This nauplius larva is probably also of Cyclops.
Darkfield, x300. |
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This Cyclops shows a heavy growth of epiphytic algae.
Darkfield, x150.
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Diaptomus.
Looking rather like a slimmer version of Cyclops, but with much longer antennae, Diaptomus is a more common inhabitant of freshwater locations in the summer and is rarely seen in the winter. It is a calanoid crustacean, and the females produce their eggs in a single egg-sac which is carried in the median position.
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This female Diaptomus is seen swimming with its large antennae rigidly extended, the smooth gliding motion being provided by the action of smaller appendages beneath the body. The males are easily identified by the modified right antenna which is used to grasp the female during mating.
Darkfield, x50. |
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Side view of a mature female Diaptomus. In this position, the antennae can be seen to extend back much further than the length of the body.
Darkfield. x65. |
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Another female Diaptomus swimming.
Darkfield, x45.
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Female Diaptomus as above seen from the side.
Darkfield, x45. |
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